The invention relates to a device for photorefractive surgery on the cornea of the eye for the correction of sight defects of a higher order.
Photorefractive keratectomy is a hitherto widely established procedure for correcting defective vision of a lower order, i.e. for example of myopia, hyperopia, astigmatism, myopic astigmatism and hyperopic astigmatism. The term xe2x80x9cphotorefractive keratectomy (PRK)xe2x80x9d is usually understood to mean that an intervention on the surface of the cornea is only intended after the so-called corneal epithelium has been removed. After removal of the epithelium the Bowman""s membrane or the corneal stroma is exposed and can be removed by a laser. The LASIK procedure (laser in situ keratomileusis) is generally distinguished from PRK. In the LASIK procedure an approximately 100 xcexcm to 200 xcexcm thick cornea slice (so-called xe2x80x9cflapxe2x80x9d) with a diameter of 8 to 10 mm is cut down to a small remnant serving as a xe2x80x9chingexe2x80x9d with a so-called microkeratome. This slice (flap) is folded to the side and ablation (removal) of material is then effected by laser radiation directly in the stroma, i.e. not on the surface of the cornea. After laser treatment the lid is folded back to its original position again and healing generally takes place relatively quickly.
The invention described below is suitable both for the above-described PRK as well as in particular the LASIK technique.
In PRK and in LASIK, corneal material is removed. The removal is a function of the lumination of the laser beam striking the cornea (energy per unit of area). Various techniques are known for beam formation and beam positioning thus, for example, the so-called slit scanning, in which the radiation is guided by means of a moved slit over the region to be treated, the so-called scanning-spot, in which a radiation spot with very small dimensions is guided over the area to be removed, and also the so-called full-ablation or wide-field ablation, in which the radiation is directed extensively over the entire area to be removed and wherein the lumination alters across the beam profile in order to achieve the desired removal of cornea. The state of the art includes suitable algorithms for controlling the radiation for the aforementioned beam positioning in each case in order to remove the cornea such that the cornea finally has the desired radius of curvature.
The aforementioned scanning-spot uses a laser beam focused on a relatively small diameter (0.1 to 2 mm), which laser beam is directed by means of a beam positioning device onto various points of the cornea and is moved successively by a so-called scanner such that ultimately the desired removal of cornea is achieved. Removal takes place therefore in accordance with a so-called ablation profile. In PRK and LASIK so-called galvanometric scanners can in particular be used (cf. Essay by G.F. Marshall in LASER FOCUS WORLD, June 1994, page 57). In the meantime other scanning techniques have been disclosed for the positioning of the laser beam.
According to the state of the art, the aforementioned types of defective vision of a lower order (for example myopia, hyperopia, astigmatism) are at present determined according to the so-called refraction data of the patient""s eye i.e. the dioptric value measured for the patient""s eye determines the ablation profile in accordance with which 5 material is removed (ablated) from the cornea (cf. T. Seiler and J. Wollensak in LASERS AND LIGHT IN OPHTHALMOLOGY, Vol. 5, No. 4, pages 199 to 203, 1993). In accordance with this state of the art, for a given patient""s eye with a specific dioptric value the laser radiation is therefore guided over the cornea such that a predetermined ablation profile corresponding, for example, to a parabola in a correction for myopia is removed. In other words: the ablation profile is adapted only in accordance with the dioptric value to the individual eye but not however in accordance with local irregularities of the optical system xe2x80x9ceyexe2x80x9d.
The essay by J. K. Shimmick, W. B. Telfair et al in JOURNAL OF REFRACTIVE SURGERY, Vol. 13, May/June 1997, pages 235 to 245 also describes the correction of sight defects of a lower order by means of photorefractive keratectomy, wherein the photoablation profiles correspond to theoretical parabolic shapes. Furthermore, it is only proposed there to incorporate some empirical correction factors into the ablation profile, which correction factors take into account the interaction between laser and tissue in order to achieve a paraboloid-shaped removal on the eye as a result.
A particular problem in photorefractive kerotectomy and LASIK is the relative positioning of laser beam and eye. The state of the art knows various processes for this thus, for example, so-called xe2x80x9ceye trackersxe2x80x9d, i.e. devices which determine the movements of the eye in order to then control (track) the laser beam used for the ablation in accordance with the eye movements. DE 197 02 335 C1 for example, describes the state of the art with regard to this.
As aforementioned above, the procedures for photorefractive cornea surgery of the state of the art for correcting defective vision of a lower order are substantially xe2x80x9call-inclusive proceduresxe2x80x9d in the sense that the correction takes account of the (all-inclusive) dioptric value of the eye. Such defective vision of a low order can, for example, be corrected with spherical or astigmatic lenses or also with a photorefractive correction of the cornea.
The optical image in the eye is however affected not only by the aforementioned types of defective vision of a lower order but also by so-called image distortions of a higher order. Such image distortions of a higher order occur in particular after operative interventions to the cornea and inside the eye (cataract operations). Such optical aberrations can be the reason why complete visual acuity (visus) is not attained despite a medical correction of a defect of a lower order. In DER OPHTHALMOLOGE, No. 6, 1997, page 441 P. Mierdel, H.-E. Krinke, W. Wigand, M. Kaemmerer and T. Seiler describe a measuring arrangement for determining the aberration of human eyes. With such a measuring arrangement, aberrations (image distortions) for monochromatic light can be measured, more specifically aberrations caused by the cornea as well as image distortions caused by the entire ocular image system of the eye can be measured and this can be done site-dependently, i.e. with a specific resolution for given sites within the pupil of the eye, it can be determined how large the image distortion of the entire optical system of the eye to be corrected is at this point. Such image distortions of the eye are mathematically described in the above-aforementioned work by P. Mierdel et al as a so-called wave-front aberration. Wave-front aberration is understood to mean the spatial course of the distance between the actual light wave-front of a central light point and a reference surface, such as, for example, its ideal, ball-shaped form. Therefore, the ball surface of the ideal wave-front, for example, serves as a spatial reference system. It is also known as such in the state of the art to select a plane as a reference system for the aberration measurement if the ideal wave-front to be measured is flat.
The measuring principle according to the aforementioned work by P. Mierdel, T. Seiler et al is also used as a starting point in the realisation of the present invention. It substantially involves a parallel beam bundle of sufficient diameter being divided by a shadow mask into separated parallel individual beams. These individual beams pass through a convex lens (so-called aberroscope lens) and as a result are focused in the emmetropic eye at a specific distance in front of the retina. The result is clearly visible projections of the mask shadows on the retina. This retinal light point pattern is depicted according to the principle of indirect ophthalmoscopy onto the sensor surface of a CCD video camera. In the aberration-free ideal eye the depicted light point pattern is not distorted and corresponds to the shadow mask pattern exactly. If there is an aberration however, there are individual displacements of each pattern point because each individual beam passes through a specific cornea or pupil region and in accordance with the irregular optical effect experiences a deviation from the ideal course. Finally the wave-front aberration is determined by a method of approximation as a site function over the pupil surface from the displacements of the retinal pattern points. The aforementioned state of the art also describes the mathematical representation of this wave-front aberration in the form of a so-called xe2x80x9cwave-front aberration mountain rangexe2x80x9d. This xe2x80x9cwave-front aberration mountain rangexe2x80x9d gives a value for the wave-front aberration W (x, y) over each pupil site (x-y coordinates), which value is then plotted as a value over the x-y coordinates. The higher the xe2x80x9cmountain rangexe2x80x9d the larger the image distortions in the eye at the respective pupil site. In a first approximation there is a proportionality between the measured deviation of the corresponding retinal light point from its ideal position and the steepness of the xe2x80x9cwave-front aberration mountain rangexe2x80x9d for each incident light beam. Thus as a result the wave-front aberration can be determined as a site function based on an arbitrary reference value on the optical axis of the system. Ideal, generally undistorted light point positions on the retina, which can supply the reference value are, for example, four central points with little mutual spacing. Such points represent a central cornea/pupil zone of approximately 1 to 2 mm diameter which, in accordance with experience, can be accepted as being generally free of image distortions of a higher order.
The xe2x80x9cwave-front aberration mountain rangexe2x80x9d can be illustrated in various ways mathematically with the aid of a closed expression (a function). Approximations in the form of a sum of Taylor or also in particular Zernike polynomials are considered in particular. The Zernike polynomials have the advantage that their coefficients are directly related to the generally known image distortions (opening defects, coma, astigmatism, distortion). The Zernike polynomials are a set of completely orthogonal functions. In an essay by J. Liang, B. Grimm, S. Goelz and J. F. Bille, xe2x80x9cObjective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensorxe2x80x9d, Optical Society of America, 11(7):1949-1957, July 1994, it is shown how the wave-front (or wave-front aberration) can be calculated from the grid point displacements. The actual wave-front can be ascertained from the determining of the derivation function of the wave-front. The wave-front emerges as a solution to an equation system. The essay by H. C. Howland and B. Howland, xe2x80x9cA subjective method for the measurement of monochromatic aberrations of the eyexe2x80x9d, Journal of the Optical Society of America, 67(11):1508-1518, November 1977, also describes a procedure for determining the monochromatic aberration and the ascertaining of the first fifteen Taylor coefficients. This state of the art can be consulted.
A measurement of the aberration and the retinal image quality of the human eye is also described in the following essay: xe2x80x9cAberrations and retinal image quality of the normal human eyexe2x80x9d, Junzhong Liang and David R. Williams, Journal Optical Society America A, Vol. 14, No. 11, November 1997, pages 2873 to 2883.
In WO 99/27334 (published after the priority date of the present application) the wave-front aberration of the eye is measured and used for the subsequent ablation.
The state of the art also already knows the attempt to ascertain ablation profiles (removal profiles) individually and site dependently for an eye to be corrected and this is based on so-called topographical measurements of the surface of the cornea, cf. C. E. Martinez, R. A. Applegate et al in ARCH OPHTHALMOL/Vol. 116, August 1998, pages 1053 to 1062. Such topographies of the surface of the cornea only supply data however on the cornea curvature, i.e.. height data at each point of the surface of the cornea. Whilst aberrations can be calculated from this data, this data does however only supply defects of a higher order on the surface of the cornea and not aberration values for the entire optical system xe2x80x9ceyexe2x80x9d. The resolution capacity of the eye (visus) is determined however not only by the surface of the cornea but also by the entire optical system of the eye to be corrected (for example the eye lens also), so an improvement is also desirable in this context.
The object of the invention, starting from this state of the art, is to provide a device for photorefractive keratectomy with which sight defects of a higher order can be treated.
For the solution of this technical problem the invention provides a combination comprising the following devices:
an aberroscope for measuring the wave-front aberration of the entire optical system of the eye to be corrected in relation to a specific eye position,
means for deriving a photoablation profile from the measured wave-front aberration in such a way that a photoablation in accordance with the photoablation profile minimises the wave-front aberration of the treated eye, and
a laser radiation source and means for controlling the laser radiation in relation to the specific eye position for the removal of the photoablation profile.
A preferred design of the device according to the invention is characterised by a device for determining an instantaneous eye position and a device for adapting the photoablation profile to the eye position.
The device according to the invention therefore serves in particular to carry out a procedure for the photorefractive keratectomy of the eye to correct sight defects of a higher order with at least the following steps:
aberroscopic measuring of the wave-front aberration of the entire optical system of the eye to be corrected in relation to a specific eye position,
deriving a photoablation profile from the measured wave-front aberration to minimise the wave-front aberration, and
photoablation by laser radiation in accordance with the photoablation profile in relation to the specific eye position.
A further procedure for the photorefractive keratectomy of the eye to correct sight defects of a higher order can also be carried out. With this procedure or with a device carrying out this procedure, both of which will be described in more detail below, the ablation profile is calculated directly from the projection of points onto the cornea and the retina. xe2x80x9cProjectionxe2x80x9d here means that a light beam of small diameter is directed onto the cornea, produces the aforementioned point there and passes from the cornea to the retina where it produces a further point. The points are light spots. A change in the curvature of the surface of the cornea can be inferred from a deviation of the position of the light spot on the retina from a desired position (the desired position corresponds to an aberration-free eye) (see below) and this ultimately represents a statement about the derivation function (in the mathematical sense) of the sought ablation profile. If this procedure is carried out with a sufficient number of light beams which are directed at different points of the eye, the derivation function of the ablation profile can be ascertained over the entire surface of the eye concerned and the ablation profile itself can then be calculated mathematically therefrom. The invention also involves the apparatus for carrying out this procedure, i.e. in particular the means for directing selected light beams with defined positions and angles of incidence, the means for measuring a displacement of the light beam on the retina in relation to the desired position and the correspondingly programmed computer for ascertaining photoablation profiles from these measurements of light beam positions on the retina.